This invention relates in general to linear electromechanical actuators and in particular to a screw-type actuator system that is fault-tolerant.
There exists a great need for fault-tolerance in fly-by-wire flight control systems, especially for the control of primary flight control surfaces such as a rudders, elevators, horizontal stabilizers and ailerons. Loss of control of one of these surfaces can lead to catastrophic results.
In a conventional electromechanical fly-by-wire flight control system, an electronic control unit (ECU) sends command signals, via wire, to an electric motor. In response to these command signals, the motor drives a linear ballscrew or Acme-threaded actuator which is linked to the control surface. Should the ballscrew or Acme-threaded actuator jam, the motor fail or a wire break, the primary control surface cannot be controlled.
Linear electromechanical actuators that are fault-tolerant are known to the prior art. A non-jamming screw actuator is disclosed in Klopfenstein U.S. Pat. No. 4,745,815. An outer ballscrew nut is threaded onto an intermediate ballscrew, which is threaded onto an inner screw. A first brake prevents the ballscrew from rotating and a second brake prevents the inner screw from rotating. When a motor rotates the ballscrew nut, the ballscrew and inner screw are moved axially as a unit. In the event of jamming between the outer nut and intermediate ballscrew, the first brake is released, allowing the intermediate ballscrew to rotate with the nut. The rotating ballscrew causes the inner screw to move axially.
Although the screw actuator is tolerant to a single fault, the motor is not. If the motor fails, so too does the system.